Uber is the most common one. Some people in HK also use 快的 (Kuai-di), which is a china app, and it only has Chinese UI I think.
You can only request private cars with Uber (UberX, UberBlack) in Hong Kong, there is no UberTaxi. Kuai-di offers taxi only.
Taxi is generally not easy to hail especially ~4-5pm local time, no matter on street or use an app. This is the time when taxi drivers change their shifts. Also unlikely to find a taxi driver who is willing to go to rural areas in peak hours.
Uber is actually not yet legal in HK, for more information: http://www.scmp.com/news/hong-kong/law-crime/article/2041039/hong-kong-uber-drivers-will-face-criminal-trial-after